Add a couple of builtin functions, and the Vm::call() method

* Builtin functions print and println have been added
* If a global lookup fails, the VM will attempt to look up a builtin
* Vm::call(fun, args) allows interrupting the current execution state
  and starting a new function instead. It will return the value left on
  top of the stack.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/obj/builtin.rs

@@ -0,0 +1,47 @@
+use crate::{obj::{prelude::*, reserved::*}, vm::signal::*};
+use maplit::btreemap;
+use once_cell::sync::Lazy;
+use std::collections::BTreeMap;
+
+pub static PRINTLN_BUILTIN_FUN: Lazy<NativeFunRef> = Lazy::new(|| NativeFun::new_obj(|_, vm, args| {
+    // TODO : use __get_attr__ when it gets added
+    let to_string = {
+        let obj_ref = &args[0];
+        read_obj!(let obj = obj_ref);
+        obj.get_attr(STR_MEMBER_NAME.sym)
+            .or_else(|| obj.get_attr(REPR_MEMBER_NAME.sym))
+            .expect("no __str__ or __repr__ member")
+    };
+    let return_value = vm.call(to_string, vec![]);
+    let str_ref: &StrRef = std::any::Any::downcast_ref(&return_value).expect("to_string to return str value");
+    {
+        read_obj!(let str_obj = str_ref);
+        println!("{}", str_obj.value());
+    }
+
+    Signal::Return
+}));
+
+pub static PRINT_BUILTIN_FUN: Lazy<NativeFunRef> = Lazy::new(|| NativeFun::new_obj(|_, vm, args| {
+    // TODO : use __get_attr__ when it gets added
+    let to_string = {
+        let obj_ref = &args[0];
+        read_obj!(let obj = obj_ref);
+        obj.get_attr(STR_MEMBER_NAME.sym)
+            .or_else(|| obj.get_attr(REPR_MEMBER_NAME.sym))
+            .expect("no __str__ or __repr__ member")
+    };
+    let return_value = vm.call(to_string, vec![]);
+    let str_ref: &StrRef = std::any::Any::downcast_ref(&return_value).expect("to_string to return str value");
+    {
+        read_obj!(let str_obj = str_ref);
+        print!("{}", str_obj.value());
+    }
+
+    Signal::Return
+}));
+
+pub static BUILTIN_OBJS: Lazy<BTreeMap<Sym, ObjRef>> = Lazy::new(|| btreemap! {
+    PRINTLN_BUILTIN_NAME.sym => PRINTLN_BUILTIN_FUN.clone() as _,
+    PRINT_BUILTIN_NAME.sym => PRINT_BUILTIN_FUN.clone() as _,
+});